The invention relates to techniques for forming heat-sealed product containers, and more particularly to techniques for registering heat-sealable, stretchable lids with mating product-filled containers prior to the heat-sealing operation.
In known arrangements of this type described, e.g., in U.S. Pat. No. 3,267,639, issued to J. G. H. OLLIER ET AL on Aug. 23, 1966, a first ductile strip carrying a plurality of product-receiving troughs preformed therein is incrementally advanced through equal first intervals into a heat-sealing station. The lids for the troughs are simultaneously advanced into the heat-sealing station, the lids being defined by spaced portions of a second ductile strip between detectable markings located at equal intervals along the second strip. The troughs are disposed at a center distance equal to the first interval, while the detectable markings on the second strip are normally spaced at second intervals which are slightly smaller than the first intervals of the troughs on the first strip.
In such known arrangement, the advancing means are disposed downstream of the heat-sealing station and positively grips the front ends of both the first and second strips. As soon as the advancing means has indexed both of such strips through the second interval, a mark on the second strip comes into registration with a photocell, which generates a signal that effects the clamping of one portion of the second strip at a single point upstream of the photocell. Since the advancing means continues to move until the entire first indexing interval has been traversed, and since the front ends of the first and second strips are firmly attached to the advancing means, the entire portion of the second strip between the clamp point behind the photocell and the gripped front end will be temporarily stretched by an amount adjusted to assure that the distance between successive markings on the portion of the second disposed at the heat-sealing station corresponds to the first interval. As a result, such portion of the second strip is registerable with the trough then entering the heat-sealing station to assure proper alignment of the container parts prior to the heat-sealing operation proper.
This arrangement has several disadvantages. For example, since the advancing means must provide the full stretching force on the second strip, such advancing means must be of rugged and expensive fabrication or else it will not provide the required elongation of the second strip by the time the advancing means comes to rest, thereby leading to registration errors at the heat-sealing station.
Moreover, since the advancing means in such apparatus is located downstream of the heat-sealing station, a portion of the first and second strips simultaneously indexed by the advancing means are already welded together behind the advancing means. The unavoidable initial fractures occurring at the welding points therefore subjects the later-stretched second strip to the risk of tearing and other defects, particularly in view of the typically long length of stretched second strip between the advancing means and the single clamping point upstream of the photocell.